In recent years, the field of plasmonics has been flourishing with new advances and discoveries. There has been tremendous development on new geometries for nanoparticles (NPs) or for nanostructured surfaces in order to control the optical properties or obtain new functionalities [1-4]. Surprisingly, the development in terms of material used for these nanostructures has been much more limited as most applications of plasmonics use either gold (Au) or silver (Ag). These two metals have the best plasmonic resonance, and although Ag has better plasmonic properties [2], Au is biocompatible and has a better stability and resistance to oxidation, making it a better candidate for several applications [5]. In either case, the control of the optical properties of the plasmonic nanomaterials can only be obtained through a fine control of the geometry using, for instance, nanorods, nanotriangles, or nanoshells rather than spherical NPs. However, the optical properties are generally sensitive to the shape of the NPs [3], making it important to obtain suspensions with good uniformity in size and shape. Therefore, it would be interesting to keep spherical NPs and to control their optical properties by changing the material composition instead of the shape. Because Au and Ag are good plasmonic metals, their alloys are a logical choice for a new material. Gold-silver (AuAg) alloy nanoparticles are interesting because their plasmonic resonance peak can be tuned with the alloy composition [6].
Synthesis of AuAg ANPs has been reported by ultrasonic alloying of Au and Ag NPs [7], by laser alloying of Au and Ag NPs [6,8], by laser ablation of a solid AuAg alloy target [5,9], by photochemical co-reduction of Au and Ag salts [10], or by using conventional chemical reduction methods in organic solvents [11-13] or aqueous solutions [14-19]. In every case, the plasmon peak position was found to vary almost linearly with alloy composition. Other ANPs syntheses are known in the art [39,40]. However, in these approaches, size control of the particles is not always achieved. Moreover, the mean diameter of the particles is generally smaller than 30 nm.
In plasmonic applications, large particles (for example >50 nm) are often needed in order to benefit from their high light scattering efficiency. For example, suitably large AuAg ANPs with different compositions can act as chromatic biomarkers in biomedical imaging [20-22].
There is a need for alloy nanoparticles having larger sizes, for example sizes larger than 30 nm. There is a need for processes for the preparation of alloy nanoparticles, which allow for the control of the particles size. Also, there is a need for processes which allow for the control of the alloy composition.